Picture perfect.

Every public penny was well spent: $9,919,370 is what it took to build the 585-acre retreat known as Prospect Park. After a little more than a year of work, the majestic space opened 150 years ago this week in the heart of what was then America’s third-largest city, Brooklyn, population 266,000.

Multiply that by 10, fast-forward through eras of automobiles and airplanes and smartphones, and one thing remains constant: The great green masterpiece designed by the legendary landscape architects Frederick Law Olmsted and Calvert Vaux brilliantly serves the people of Brooklyn, an oasis of sanity and calm in an urban landscape with too little of either.

When designing Central Park in Manhattan a decade before, the pair had been forced to tussle with what Olmsted called “infernal scoundrels,” political types whose meddling parochialism prevented them from bringing their vision to life. (Their descendants still roam.)

Brooklynites promised the pair that if they crossed the river, they’d have free rein.

The resulting jewel is a breathing place of peace where the people of what is the city’s most populous borough — new and native-born New Yorker; poor, middle-class and rich — can rest and read and play and exercise and fly kites and hear concerts and have kiddie birthday parties, and be together, or alone together.

There are twisting paths, hills and ravines, and large meadows that suddenly appear. Bikeways. Playgrounds. The shelter of thousands of trees. A carousel. A pond. A snazzy new splash pad and roller rink opened three years ago as part of an ongoing revitalization.

The park has fallen on hard times, as has the city and borough it constantly helps replenish. In the late 1980s, the Prospect Park Alliance rushed to the rescue, bringing pride, private money and management smarts that have served it well to this day.

Those who know Prospect Park well consider it a friend. Now, it is an old, old friend — but one that continues to dazzle with its beauty. It is warm and familiar, yet it never fails to surprise.